1. Field of the Invention
This invention relates to an image-forming apparatus and an image-forming method which make use of an amorphous-silicon electrophotographic photosensitive member, a contact charging means and a spherical toner.
2. Related Background Art
Conventionally, it is common to use corona charging assemblies in charging units for photosensitive members used in, e.g., plain-paper copying machines, laser beam printers, LED printers and liquid-crystal shutter printers, and such corona charging assemblies are in wide use. The corona charging assemblies charge object members electrostatically by applying a high voltage of about 5 to 10 kV to a metal wire of about 50 to 100 xcexcm in diameter to ionize the atmosphere.
For structural reasons, the corona charging assemblies have a disadvantage that generation of ozone in a large quantity accompanies corona discharging. With their repeated used, ozone and corona products may become deposited on the photosensitive member surface, under the influence of which the photosensitive member surface may become susceptible to humidity to tend to absorb moisture content. This may cause a lateral flow of electric charges on the photosensitive member surface in an environment of high temperature and high humidity to cause a lowering of image quality which is called smeared images. In particular, electrophotographic photosensitive members making use of amorphous silicon (hereinafter xe2x80x9ca-Si photosensitive memberxe2x80x9d) have so high a surface hardness that, while they are durable to printing on a large number of sheets, their surfaces may abrade with difficulty. Hence, corona products having once adhered can be removed with difficulty to have a great influence.
The corona charging assemblies are also usually often used under constant-current control. In such a case, they tends to be affected by any uneven layer thickness and resistance distribution of the photosensitive member. This may cause unevenness in surface potential, and may consequently cause uneven density on images.
In order to solve such a problem on image quality, various charging units are proposed.
In a contact charging unit as disclosed in Japanese Patent Application Laid-open No. 63-208878, a charging member to which a voltage is kept applied is brought into contact with an object member to be charged (photosensitive member), which is called charging object member, to charge the photosensitive member surface to an intended potential. Compared with the corona charging assemblies, such a unit can achieve a low voltage in respect of the applied voltage necessary for providing the desired potential on the charging object member surface, and does not cause any smeared images due to the ozone products because the quantity of ozone occurring in the course of charging is zero or is very small. Also, in such contact charging, the surface of the photosensitive member is charged to have substantially a uniform potential in accordance with the applied voltage, and hence uneven image density may little occur. It has such advantages.
In the way of progress such that a series of contact charging members are improved in various manners, as disclosed in Japanese Patent Application Laid-open No. 8-6353, a mechanism is proposed in which a contact charging member making use of particles in the form of a magnetic brush comprised of a magnetic material and magnetic particles (or powder) is brought into contact with an electrophotographic photosensitive member to provide it with charge. Also proposed is, as disclosed in Japanese Patent Application Laid-open No. 10-307454, a new method of a mechanism in which a carrying member having conductivity and elasticity so constructed that charged particles are carried on the surface is brought into contact with a photosensitive member to provide it with charge.
Attempts to achieve much higher image quality are also made from improvements of toners. More specifically, polymerization toners are on studies in place of conventional pulverization toners.
The polymerization toners have superior fluidity because they have particles in substantially a uniform spherical shape and having less scattering in particle diameter. Also, they are advantageous to the achievement of high image quality because they do not let colorants come bare to particle surfaces and have uniform triboelectric chargeability. Still also, they can enclose wax in particles, and can attain good fixing performance and anti-offset properties. Hence, the polymerization toners are being gradually widely employed in high-image-quality machines. As a patent application which proposes a magnetic polymerization toner, EP1058157 A1 is accessible.
In recent years, what also attracts notice is to make image-forming apparatus small-sized. In image-forming apparatus, usually a latent image is developed with a toner to make it into a visible image, the toner image is transfer to a transfer medium such as paper, and thereafter toner particles having remained on a photosensitive member without being transferred onto the transfer medium are removed through a cleaning step. With regard to this cleaning step, blade cleaning, fur brush cleaning, roller cleaning and so forth have conventionally been used. However, from the viewpoint of apparatus, apparatus are necessarily set up in a large size because a unit for such cleaning must be provided. This has been a bottleneck in making apparatus compact.
In addition, from the viewpoint of ecology, the waste toner that comes from the cleaning step is undesirable. In the sense of effective utilization of toners, too, it has been sought to provide a system which does not send forth any waste toner.
As one means for meeting such demands, an image-forming apparatus employing the technique called cleaning-at-development or cleanerless. The cleanerless image-forming apparatus is an apparatus in which any conventional cleaning unit is not provided and the transfer residual toner having remained on the surface of an electrophotographic photosensitive member is collected at its developing means which performs development simultaneously. Employment of this technique makes it possible to save the space for the part of the cleaner, and can contribute towards making image-forming apparatus compact. Also, since any waste toner does not come out, such apparatus have the merit of being tender of environment and improving utilization efficiency of toners.
As stated above, attempts to achieve much higher image quality are being made by combining the formation of uniform latent images free of any unfocused or uneven images that is attributable to contact charging units with the formation of faithful visible images that is attributable to polymerization toners.
However, in the case when the voltage application type contact charging unit is utilized as a means for charging the electrophotographic photosensitive member, there are the following problems.
In such a contact charging unit, it has very good charge potential uniformity when viewed macroscopically as stated above. However, when viewed microscopically, for the reasons of its construction, marks of contact of the magnetic brush or charged particles with the photosensitive member (brush images) may appear. In such a case, it is necessary to make higher the relative speed between the charging unit and the photosensitive member to make them rub against each other in a greater extent so that the charging unit can be brought into uniform contact with the electrophotographic photosensitive member. However, because of such rubbing, the surface of the photosensitive member may abrade or wear, though slightly. Although such wear is at a small level, even microscopic abrasion may have a great influence when it lasts over a long period time, because the a-Si photosensitive member has a long lifetime originally. Accordingly, it is a subject how the contact performance be improved while the abrasion level of the photosensitive member surface is reduced.
As another problem other than such uneven charging, there is also a problem that the contact charging units deteriorate. For example, in the case of a magnetic-brush type contact charging assembly, its magnetic particles may migrate to the electrophotographic photosensitive member side, which is a problem of what is called magnetic-particle leakage. Once the contact charging unit has deteriorated in this way, faulty charging may occur or image deterioration may occur. Hence, this provides a subject on how the contact charging units be made to have long lifetime.
In the case of the image-forming apparatus having cleanerless construction, there is also a subject how the transfer residual toner be collected in the developing assembly. Because of such transfer residual toner, image fog inevitably tends to occur greatly, compared with conventional image-forming apparatus having a cleaner. Accordingly, it has been sought to make more improvement.
The problem of image fog in this cleanerless image-forming apparatus tends to become severer as copying process becomes higher. Accordingly, it has been sought to provide an image-forming apparatus that can meet the demand for higher speed in recent years.
With regard to the polymerization toner, although it has so good a transfer efficiency as to send forth less transfer residual toner, it is difficult for the transfer residual toner on the photosensitive member to be well removed from the photosensitive member surface by the aid of a cleaning blade. Hence, the transfer residual toner may remain on the photosensitive member surface even after cleaning. This is because, the toner has so a uniform particle surface shape that it has a high rolling action mutually between the cleaning blade, the photosensitive member and the toner, so that the toner is not well scraped off by the cleaning blade. It is true that the transfer residual toner can well be removed to a certain extent by bringing the cleaning blade into touch with the photosensitive member surface at a higher pressure to strengthen the action of mechanical scraping. In such a case, however, there have been problems that the photosensitive member is worn by the cleaning blade or the blade turns over. Also, the toner may melt-adhere to the photosensitive member surface or may cause filming thereon to cause a problem that it is difficult to make the photosensitive member have a higher running performance and form images at a higher process speed.
Many proposals are also made on the improvement of photosensitive members themselves. As a patent application concerning an a-Si photosensitive member having a surface layer formed of a non-single-crystal carbon film, Japanese Patent Application Laid-open No. 11-184121 is accessible.
An object of the present invention is to provide an image-forming apparatus and an image-forming method which have overcome the above problems.
Stated more specifically, an object of the present invention is to provide an image-forming apparatus and an image-forming method which are able to obtain high-quality images free of any unfocused images and smeared images in every environment, without causing any generation of ozone products due to corona discharging.
Another object of the present invention is to provide an image-forming apparatus and an image-forming method in which the a-Si photosensitive member can uniformly be charged to obtain uniform images free of any uneven images and also free of any brush images or coarse images in halftone images.
Still another object of the present invention is to provide an image-forming apparatus and an image-forming method in which the a-Si photosensitive member does not wear and operates stably over a long period of time.
A further object of the present invention is to provide an image-forming apparatus and an image-forming method in which the contact charging unit has a long lifetime and images can stably be obtained at a minimum maintenance cost and over a long period of time.
A still further object of the present invention is to provide an image-forming apparatus and an image-forming method which promise a high image quality and in which, even when the polymerization toner is used, good cleaning performance can be maintained, without causing difficulties such as melt adhesion, filming and also wear of photosensitive members.
The present inventors have made extensive studies on the achievement of higher image quality in image-forming apparatus making use of a-Si photosensitive members. As the result, they have reached a conclusion that it is effective to use a contact charging type charging assembly in order to be free of the smeared images and uneven charging that are questioned when the a-Si photosensitive member is charged by means of a corona charging assembly, and also to use a polymerization toner in combination in order to form sharp images in a high resolution.
That is, the present invention provides an image-forming apparatus comprising:
an electrophotographic photosensitive member having a conductive substrate, and at least a photoconductive layer and a surface layer on the conductive substrate;
a charging means for charging the electrophotographic photosensitive member electrostatically;
a latent-image-forming means for performing imagewise exposure to form an electrostatic latent image on the electrophotographic photosensitive member;
a developing means for moving a toner to the electrostatic latent image formed on the electrophotographic photosensitive member, to render the electrostatic latent image visible to form a toner image; and
a transfer means for transferring the toner image to a transfer medium;
wherein;
the photoconductive layer of the electrophotographic photosensitive member comprises a non-single-crystal material composed chiefly of silicon;
the surface layer of the electrophotographic photosensitive member comprises a non-single-crystal carbon film containing at least hydrogen, and has an arithmetic-mean roughness Ra ranging from 0 nm to 100 nm in an extent of 10 xcexcmxc3x9710 xcexcm of the surface layer;
the charging means is a magnetic-brush charging assembly for charging the electrophotographic photosensitive member electrostatically upon application of a voltage, bringing a magnetic brush formed by binding magnetic particles magnetically into contact with the surface of the electrophotographic photosensitive member; and
the toner is a magnetic toner having toner particles containing at least a binder resin and a magnetic material, and an inorganic fine powder, and having an average circularity of from 0.950 to 1.000;
the toner having a saturation magnetization of from 10 to 50 Am2/kg (emu/g) under application of a magnetic field of 79.6 kA/m (1,000 oersteds).
The present invention also provides an image-forming method comprising:
a charging step of electrostatically charging an electrophotographic photosensitive member having a conductive substrate, and at least a photoconductive layer and a surface layer on the conductive substrate;
a latent-image-forming step of performing imagewise exposure to form an electrostatic latent image on the electrophotographic photosensitive member;
a developing step of moving a toner to the electrostatic latent image formed on the electrophotographic photosensitive member, to render the electrostatic latent image visible to form a toner image; and
a transfer step of transferring the toner image to a transfer medium;
wherein;
the photoconductive layer of the electrophotographic photosensitive member comprises a non-single-crystal material composed chiefly of silicon;
the surface layer of the electrophotographic photosensitive member comprises a non-single-crystal carbon film containing at least hydrogen, and has an arithmetic-mean roughness Ra ranging from 0 nm to 100 nm in an extent of 10 xcexcmxc3x9710 xcexcm of the surface layer;
the charging step is a charging step making use of a magnetic-brush charging assembly for charging the electrophotographic photosensitive member electrostatically upon application of a voltage, bringing a magnetic brush formed by binding magnetic particles magnetically into contact with the surface of the electrophotographic photosensitive member; and
the toner is a magnetic toner having toner particles containing at least a binder resin and a magnetic material, and an inorganic fine powder, and having an average circularity of from 0.950 to 1.000;
the toner having a saturation magnetization of from 10 to 50 Am2/kg (emu/g) under application of a magnetic field of 79.6 kA/m (1,000 oersteds).
The present invention still also provides an image-forming apparatus comprising:
an electrophotographic photosensitive member having at least a conductive substrate, and a photoconductive layer and a surface layer which are superposingly formed on the conductive substrate;
a charging means for charging the electrophotographic photosensitive member electrostatically;
a latent-image-forming means for performing imagewise exposure to form an electrostatic latent image on the electrophotographic photosensitive member;
a developing means for rendering the electrostatic latent image visible by the use of a toner to form a toner image; and
a transfer means for transferring the toner image to a transfer medium;
wherein;
the photoconductive layer of the electrophotographic photosensitive member comprises a non-single-crystal material composed chiefly of silicon;
the surface layer of the electrophotographic photosensitive member comprises a non-single-crystal carbon film containing at least hydrogen, and has an arithmetic-mean roughness Ra ranging from 0 nm to 100 nm in an extent of 10 xcexcmxc3x9710 xcexcm of the surface layer;
the charging means has a conductive fine powder and a charging member holding the conductive fine powder on its surface; the conductive fine powder forming the part of contact with the electrophotographic photosensitive member; and is a charging means for charging the electrophotographic photosensitive member electrostatically upon application of a voltage to the charging member; and
the toner is a magnetic toner having toner particles containing at least a binder resin and a magnetic material, and an inorganic fine powder, and having an average circularity of from 0.950 to 1.000;
the toner having a saturation magnetization of from 10 to 50 Am2/kg (emu/g) under application of a magnetic field of 79.6 kA/m (1,000 oersteds).
The present invention further provides an image-forming method comprising:
a charging step of electrostatically charging an electrophotographic photosensitive member having a conductive substrate, and at least a photoconductive layer and a surface layer on the conductive substrate;
a latent-image-forming step of performing imagewise exposure to form an electrostatic latent image on the electrophotographic photosensitive member; a developing step of rendering the electrostatic latent image visible by the use of a toner to form a toner image; and
a transfer step of transferring the toner image to a transfer medium;
wherein;
the photoconductive layer of the electrophotographic photosensitive member comprises a non-single-crystal material composed chiefly of silicon;
the surface layer of the electrophotographic photosensitive member comprises a non-single-crystal carbon film containing at least hydrogen, and has an arithmetic-mean roughness Ra ranging from 0 nm to 100 nm in an extent of 10 xcexcmxc3x9710 xcexcm of the surface layer;
the charging step is a charging step of applying a voltage to a charging member to charge the electrophotographic photosensitive member electrostatically by means of a charging member holding a conductive fine powder on its surface; the conductive fine powder forming the part of contact with the electrophotographic photosensitive member; and
the toner is a magnetic toner having toner particles containing at least a binder resin and a magnetic material, and an inorganic fine powder, and having an average circularity of from 0.950 to 1.000;
the toner having a saturation magnetization of from 10 to 50 Am2/kg (emu/g) under application of a magnetic field of 79.6 kA/m (1,000 oersteds).